This invention relates to prosthetic devices used to replace all or part of the ossicular structure of the middle ear.
In the human ear, sound entering the external auditory canal strikes the tympanic membrane, or ear drum, causing the membrane to vibrate. The vibrations of the tympanic membrane are transmitted through three ossicles in the middle ear, known as the malleus, incus and stapes, to an opening in the bony wall of the inner ear called the oval window. In the inner ear, the vibrations are transduced to nerve impulses that are deciphered in the brain. When the ossicular structure of the middle ear is removed or damaged, hearing is lost.
A variety of prostheses has been developed to replace the ossicular structure of the middle ear. Typically, the prosthesis is a one piece device with a head portion that is adapted to fit against the tympanic membrane and a shaft adapted to bridge the gap across the middle ear to contact at its distal end the oval window, or any remaining portion of the ossicular structure. Several such devices are shown in U.S. Pat. Nos. 3,473,170; 3,909,852 and 4,052,754.
The prostheses were found to perform best when they were constructed from a material that would permit tissue ingrowth from the tympanic membrane to anchor the head portion of the device. Likewise, when the prosthesis contacted the oval window, tissue in growth could anchor the distal end of the shaft. A porous polyethylene sold by Richards Medical Company under the name "PLASTI-PORE" as well as other porous polymeric materials became the preferred materials for use in such prosthesis because they possessed the requisite strength and density for performing the sound conducting function of the prosthesis, as well as the requisite porosity and biocompatibility to promote tissue ingrowth.
However, because the angle of the tympanic membrane with respect to the oval window varies from person to person, prosthesis constructed from the rigid, porous, high-density polymeric materials would not always fit well. A poor fit could affect the sound transmission efficiency of the device, or in some instances, lead to dislodgement or cause damage to the tympanic membrane or inner ear. Bending a prosthesis made of porous, high-density polymeric material caused uneven stress on the tissues of the ear, or even crumbling of the prosthesis. To overcome this problem, a prosthesis was developed by Richards Manufacturing Company, the corporate predecessor to the owner of the instant invention, that included a joint between the shaft and the head portion so that the head could be titled to conform to the angle of the tympanic membrane. This improved device is shown in U.S. Pat. No. 4,281,419. However, the movable joint required separate parts that could disarticulate and was relatively difficult to manufacture.